The subject invention relates to a method and device for measuring a distance from the device to an object using light, and, more specifically, to a method and device for measuring a distance from the device to an object wherein light is emitted from one or more light sources of the device and reflected from a surface of the object to one or more light detectors of the device. The light travels over paths of differing length between the light source(s), the object reflecting the light, and the light detector(s). The light intensity measured by the light detector(s) generates electrical signals that are used to calculate the distance from the device to the object.
There are various applications in which it is desirable to use light to measure distances. Examples of such applications include measuring a distance between a camera and a subject for auto focusing, measuring distances during surveying, measuring astronomical distances, etc. Correspondingly, a variety of methods that use light for measuring distances have been described. One method for measuring distances with light is to measure the time for a light pulse to travel between two points. For example, U.S. Pat. No. 5,532,813, Ohishi et al, discloses a method for measuring distances using lasers wherein a distance-measuring means calculates relatively long distances, e.g. kilometers in distance, to a target based on the time difference between a light beam emission and the reception of a return light beam from the target. However, the lasers and optics in such approaches are too complex and expensive for budget limited or highly miniaturized applications. Other, lower cost methods for the time of flight measurement of distances have been described. For example, U.S. Pat. No. 5,701,006, Schaefer, discloses a method for measuring distances in which Light Emitting Diodes (LEDs) and photodetectors are used in fiber optics, and modulation frequencies in the MHz range are used to measure time delays. Although such approaches use lower-cost LEDs and photodetectors, relatively short, properly shaped pulses are required. Electronics in the radio frequency range must also be used to process the signals. These factors introduce higher levels of noise into the overall system, thus relatively complex electronics circuitry or specialized, high power pulse generation at the LED is needed.
For applications such as camera focusing, it is common to transmit a light spot with an LED, then receive an image of the spot at the receiver. By measuring the location of the spot on the received image, the distance to the subject can be determined by triangulation. The basic approach is subject to problems with range and reliability, and various improvements have been described in the following patents. For example, U.S. Pat. No. 5,541,723, Tanaka, discloses a method of measuring distances that involves the transmission at different angles of two differing light distributions. In this method, the information on the amount of signals received tells how much signal does not impinge on the object of interest. Nakanishi, et al., in U.S. Pat. No. 5,963,309, shows multiple LEDs and photodetectors that are used to increase the range and resolution. Kindaichi, in U.S. Pat. No. 6,172,375, uses two spaced lens that form images. From these images, positions are measured and calculated for the purpose of increasing the reliability of measurements. However, these approaches still require the use of lenses in order to obtain images of light spots or patterns. The need for lenses increases the bulk as well as the cost of a distance-measuring system.